Generally my position with these things is if something doesn't impact me then I'm not bothered. So if the tablet is on silent or headphones are being used then crack on.
However, it is undeniable that there has been a massive shift in how people parent. Small children are expected to fit into an adult centric experience now, whereas 20-30yrs ago, dining out wasn't something done all that frequently for the majority. Granted, I understand there's no real avoiding it when on holiday, it's just a general observation. I see a lot of parents dining or drinking together pretty much to the complete exclusion of their children who are expected to behave responsibly whilst enduring what must be incredible boredom for them.
And because there's little interaction, these children aren't actually learning how to behave properly, and aren't developing the requisite social skills to carry them through such situations later in life. I don't blame the children at all, I blame parents who don't prioritise the childrens needs.
As for screen use amongst adults, it's incredibly rude and quite sad. If you're out with your partner, friends or family I'd expect you would want to enjoy their company, and I don't see why a phone should factor into that. But people are largely addicted to smartphones, so just leaving them alone is really difficult as social media has trained people to think they need to know exactly what's going on all of the time.
And mindless eating, whether looking at a tablet or a TV, is proven to be a huge factor in overeating and subsequent obesity rates. It isn't a healthy way to eat and I really don't think any parent should encourage these behaviours.